Frequency modulation receiver



Patented May 27, 1941 UNITE STATES dATEN'i @FFICE rREcUENor MonUnArioNReceives Application April 13, 1940, Serial No. 329,473

6 Claims.

This invention relates to receivers for frequency modulation systems andparticularly to demodulation and conversion circuits therefor. An objectof the present invention is to provide eflicient, accurate andundistorted detection of frequency modulated waves.

In one method of detecting or demodulating frequency modulated Waves toobtain the modulating signal waves, the frequency modulations are firstconverted to amplitude modulations from which the signal may be obtainedby rectification or the like. This conversion process may beaccomplished by impressing the frequency modulated Waves on a so-calledslope circuit, that is, a circuit having an impedance which varies withfrequency. For the emcient, accurate and undistorted operation of saidcircuits several characteristics are necessary. Among these are thefollowing: First, the impedance at one limit of the frequency swingshould be low and substantially zero to provide for efiicientconversion; second, the variation of impedance must be substantiallylinear throughout the complete range of frequency variation to give anaccurate reproduction of the modulation; third, the impedance of thecircuit should fall off rapidly beyond the other limit of the frequencyvariation so that harmonic effects will be discriminated against.

Two types of circuit which have been found very useful as slope circuitsfor such purpose are the series and shunt resonant circuits. When suchcircuits are used in an amplifier the gain thereof will vary withfrequency in ac cordance with the impedance characteristic of thecircuit. Of these two types both have certain advantages anddisadvantages. Thus, the series resonant circuit will in general befound to have a linear characteristic over a wider frequency range thanthe shunt of antiresonant type.

On the other hand, in its practical forms, this circuit has acomparatively low impedance in the range of best linearity so that itcannot advantageously be used with vacuum tube amplifiers. Also theimpedance tends to increase beyond the upper limit so that harmonics areaccentuated rather than discriminated against. Shunt resonant circuitson the other hand have comparatively high impedance in the range of goodlinearity which makes them readily adapted'for use with vacuum tubeamplifiers. In addition the increase in impedance is abruptlyinterrupted at or near the resonant point so that they afiord gooddiscrimination against harmonics. However, the range of linearity iscomincluding an amplifier, a shunt resonant circuit tuned to a frequencyat or near one limit of the frequency swing of the frequency modulationsand a negative feedback path for the amplifier having such an impedancecharacteristic that the linearity of the over-all characteristic of thecircuit is increased and the impedance thereof made low in the region ofthe other frequency limit. More specifically the negative feedback pathcomprises a second shrmt resonant circuit connected in a portion of thecathode path common to both the grid and plate circuits and tuned to afrequency at or near the limit of the frequency swing, opposite to thelimit at which the first circuit is resonant.

These and other objects and features of the invention may be morereadily understood by reference to the following description of thedrawing in which:

Fig. 1 is a schematic circuit diagram of a conversion circuit of thepresent invention;

Fig. 2 is a graph illustrating the operation of the circuit of Fig. 1;

Fig. 3 is a schematic circuit diagram of a frequency modulation radioreceiver embodying the invention; and

Fig. 4 is a graph illustrating the operation of the circuit of Fig. 3.

Fig. 1 shows one embodiment of the invention in a conversion or slopecircuit for frequency modulated Waves. This circuit comprises a vacuumtube iii. The input frequency modulated Waves are supplied to the gridcircuit of this tube at the input terminals H. cludes a shunt resonantcircuit l2 connected in series with the plate battery l3 between theanode and cathode. The output amplitude modulated Waves are taken offthe secondary winding I l coupled to the inductance of the circuit Hi.In the cathode path of the tube there is connected a second shuntcircuit [5. This circuit The plate circuit in-- I5 is common to both theplate and grid circuits and therefore provides a negative feedback path.

The operation of this circuit can be readily understood by reference toFig. 2 which is a graph of the frequency-gain characteristic of thecircuit. Curve 16 shows the characteristic with only the resonantcircuit l2 operative, the circuit l5 being short-circuited by means of aswitch l8.

An examination of this curve (it) will show that the portion of thefrequency range over which the gain variation is linear is relativelylimited and particularly that at the range of lower gains the departuresfrom linearity are very marked.

Curve ll shows the characteristics of the complete circuit (switch 18open). The over-all characteristics as shown by curve l'l will beobserved to give a gain variation. with frequency which is much morelinear than that of curve It and further that it goes to a very lowvalue of gain within the range of substantially linear variation.

In the circuit on which the data for the curves of Fig. 2 were taken thecircuit 12 was tuned to 427 kilocycles employing an inductive coil of500 microhenries and a condenser of 260 micromicrofarads, the value forthe Q of the circuit or the ratio of reactance to resistance being 110'and the circuit I5 was tuned to 450 kilocycles employing an inductancecoil of 13 microhenries and a condenser of .0104 microfarad, the valueof Q being 52.

These circuit constants and the resultant characteristics are given asone typical example of what may be accomplished with the circuit of thisinvention. As will clearly appear to one skilled in the art, the variouscircuit constants both as to their absolute and relative values may bevaried to meet the design requirements in any particular application ofthe invention.

Fig. 3 shows schematically a frequency modulation radio receiver of thepush-pull type employing the conversion circuit of this invention. Inthis circuit the frequency modulated waves received in the antenna 2|are supplied to the input of a first detector 22 to which are alsosupplied oscillations from a beating oscillator 23. The intermediatefrequency output of the detector 22 is supplied to an intermediatefrequency amplifier 24 for selective amplification and then to anamplitude limiter 25. The foregoing portion of the receiver is of courseconventional in its arrangement and is therefore shown in blockschematic.

The output of the limiter 25 is supplied to a push-pull or balancedconversion circuit 26 each half of which employs a conversion circuit ofthe type shown in Fig. 1.

As is well understood in the art the operation of a push-pull converterand detector for frequency modulated waves is achieved by the use of twoslope circuits onehaving an impedance or gain characteristic whichdecreases with frequency in the range of the frequency swing and theother having the reverse slope, that is, a gain or impedance increasingwith frequency. The two curves should cross at the mean or carrierfrequency. When the outputs of such circuits are combined in phaseopposition the resultant will be zero for inputs of the main carrierfrequency. However, since the envelopes of the modified waves from thetwo conversion circuits vary in opposite senses and the combination isproduced in phase opposition the resultant components will be additiveat other frequencies.

One converter path comprises a vacuum tube amplifier 36 having a shuntresonant circuit 32 connected in its plate circuit and a second shuntresonant circuit 35 connected in the cathode path common to both theplate and grid circuits. A resistor-condenser circuit 39 is alsoconnected in this cathode path for providing a grid biasing voltage.

The other converter path is similarly arranged including the amplifiertube 45), the shunt resonant circuits 42 and 45 and the biasing network49. Plate current and screen grid bias for both tubes 30 and 40 aresupplied from the battery 2'1.

The output of the two paths are taken off through the secondary coils 34and 44 coupled to the inductance elements of the respective resonantcircuits 32 and 42. These outputs are supplied to the respective diodedetectors 33 and 43 and the resultant signal voltages developed acrossthe load resistors 38 and 48 are combined phase opposition in the signaloutput circuit Fig. 4 is a graph showing the operating characteristicsof the two converter paths comprised by the amplifiers 3i] and 46 andtheir respective circuits. In this figure, frequency is plotted againstthe ratio of output to input, that is, gain. in is the frequency of theintermediate frequency carrier and f1 and f2 the limits of the frequencyswing. The curve 3'! shows the characteristic of the amplifier 38 andthe curve 41 that of amplifier 48. To obtain such characteristics thecircuit 32 is tuned to the frequency f2 and circuit 35 is tuned to thefrequency ,fi. Similar circuits 42 and 45 are tuned to the frequenciesf1 and f2, respectively.

What is claimed is:

1. A converter circuit for a receiver of frequency modulated wavescomprising an amplifier having input and output circuits, a tunedcircuit in one of said circuits, and a feedback path for said amplifierhaving such an impedance characteristic as to cooperate with theimpedance characteristic of said tuned circuit to produce an overallgain for said amplifier that varies linearly with frequency fromsubstantially zero to a high gain across the range of the frequencyswing of said frequency modulated wave.

2. A converter circuit according to claim 1 in which the feedback pathincludes a second tuned circuit resonant at a frequency different fromthe resonant frequency of the first tuned circuit.

3. A converter circuit for a receiver of frequency modulated wavescomprising an amplifier having input and output circuits and a thirdcircuit common to said input and output circuits, a first tuned circuitconnected in said output circuit, and a second tuned circuit connectedin the common circuit, the resonant frequencies of said tuned circuitsbeing of the order of the respective limiting frequencies of thefrequency swing of said frequency modulated wave whereby the over-allcharacteristic of said amplifier is linearly proportional to frequencyover the range of said frequency swing.

4. A converter circuit for a receiver of frequency modulated wavescomprising an implifier having input and output circuits and a thirdcircuit common to said input and output circuits, a shunt type resonantcircuit included in said output circuit and tuned to a frequency of theorder of one limiting frequency of the frequency swing of the modulatedwave, and a second shunt type resonant circuit included in said thirdcircuit and tuned to a frequency of the order of the other limitingfrequency of said frequency swing.

5. A receiver for frequency modulated waves comprising two amplifierseach having an input circuit, an output circuit and a third circuitcommon to said input and output circuits, a shunt type resonant circuitincluded in the output circuit of one of said amplifiers, and tuned to afrequency of the order of one limiting frequency of the frequency swingof said modulated waves, a second shunt type resonant circuit includedin the said third circuit of said one of said amplifiers and tuned to afrequency of the order of the second limiting frequency of saidfrequency swing, a third shunt type resonant circuit included in theoutput circuit of the other of said amplifiers and tuned to a frequencyof the order of said second frequency limit, a fourth shunt typeresonant circuit included in the said third circuit of said otheramplifier and tuned to a frequency of the order of said one limitingfrequency, means for supplying the received frequency modulated waves tothe input circuits of said amplifiers in parallel, and means forcombining in phase opposition the outputs from said amplifiers.

6. A radio receiver according to claim 5 in which the last-mentionedmeans comprises means for rectifying the outputs from each of saidamplifiers and combining the rectified components in phase opposition.

EDWIN A. KRAUTH.

